The vessel of the fuel-element container has a thick-walled body made of nodular cast iron. A specific grade of this nodular cast iron which can be used is identified in German nodular cast iron specifications as GGG-40. Nodular cast iron is selected because of its especially high strength and toughness.
After the fuel element vessel is filled with irradiated nuclear reactor fuel elements, the vessel must be closed with a cover so as to be gas-tight with respect to the ambient. A tight closing of the metal container is possible by welding to it a metal cover.
However, if a cover is welded to a fuel-element vessel made of nodular cast iron, micro fissures can occur in the structure of the cast iron which can permit unwanted radioactive leakage to the ambient. To correct for such micro fissures, the container loaded with fuel elements must be subjected to a heat treatment in the temperature range of from 500.degree. to 700.degree. Centigrade. A heat treatment is generally not possible or only possible under very severe conditions since the fuel elements in the container must not be subjected to a temperature greater than 400.degree. Centigrade. In addition, it would require a major engineering effort to subject the containers weighing approximately 100 tons to a heat treatment operation. It is for these reasons that the fuel-element containers made of nodular cast iron were previously closed with cover systems incorporating threads with seals interposed.